The invention relates to an apparatus for connecting two workpieces together by a welded seam, in which the workpieces are held between opposed moving tracks or chains comprising a plurality of members, running around guide rollers in a conveying direction.
Various welding processes are known for joining two workpieces together, particularly for the fabrication of body components for motor vehicles. Especially worthy of note is laser welding in which the sheet-metal parts are butt-welded to form compound panels.
In the nature of things, the edges of the sheets which are to be welded together are not absolutely straight, and have a certain amount of waviness. This means that the butt joint is closed only where the wavy edges are in contact; at other points the edges stand back from one another, leaving the joint open. According to the industry standard in Europe for laser welding, the gap should not exceed a width of 0.08 mm at any point. This very small tolerance makes it necessary to prepare the edges of the sheets for welding. This is usually done by trimming by laser beam. However, it may also be done by a process according to EP-A 0565846, in which the sheets are butted together and at least one edge is then plastically deformed against the other, closing the gap so that its maximum width no longer exceeds 0.08 mm.
A welding operation usually has the following cycle.
The workpieces are butted together in pairs, without prepared edges, positioned without significant sideways pressure, and fixed by means of clamp elements. At this point the workpieces are first subjected to force. The workpieces may then he processed, for example they may be dressed as described in EP-A 0565846, to bring the gap within the permitted tolerance. This processing causes a second application of force to the positioning system, in which considerable lateral forces can occur. Finally, further mainly sideways acting force effects occur as a result of thermal stresses in the laser welding process.
These forces are capable of affecting the joint gap, particularly if the tolerances in the machine allow relative movement of the sheets to be welded.
A machine for joining workpieces together by means of a laser beam is known from U.S. Pat. No. 5,328,083. This machine is divided into four zones. In the first zone, a first workpiece is positioned. In the second zone, a second workpiece is brought laterally up to the first workpiece so that the two workpieces form a butt joint. In a third zone, the workpieces are then fed together to a welding station, ie. they are made to pass under a laser beam. Here the two workpieces are joined together by a welded seam. In a fourth zone, discharge of the workpieces, now welded together, takes place.
The pass of the first workpiece through the second and third zones and the pass of the second workpiece through part of the second zone and the third zone take place between opposed moving tracks. These run at constant speed, so that the feed of the workpieces to the tracks also takes place under constant speed conditions. The workpieces are grasped and "snatched away" by the tracks. This action is very abrupt, as the clamping must be capable of accepting the abovementioned lateral forces, and operates with correspondingly large forces. Positioning problems arise through relative movement of the workpieces until they are completely gripped by the tracks.
Furthermore, because one workpiece is positioned first and then the second workpiece is positioned and aligned with the first workpiece, the overall length of the machine is rather large. Moreover, a certain distance has to be maintained between successive pairs of workpieces, and this increases the unproductive time during which no welding can be done. To a certain degree this could be remedied by making the feed conveyor run faster. However, this results in increased transfer difficulties.